Vision in echolocating bats - Fladdermus.net

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The brown long-eared bat Plecotus auritus responded to patterns equivalent to 30’ of arc, which means that this species should be able to detect objects as small as 0.9 cm at a distance of 1 m. Among the Vespertilionidae, only Antrozous pallidus has been shown to have a better resolving power (15’; Bell & Fenton 1986). These results and the fact that P. auritus has larger eyes than most Vespertilionids (Cranbrook 1963; Tab 1.) suggest that it should be possible for long-eared bats to detect prey sized objects visually. It typically feeds on relatively large insects including many moths and beetles (Swift & Racey 1983, Rydell 1989). As P. auritus is a gleaner and sometimes takes insects from leaves (Swift 1998), it faces potential problems with clutter and therefore use other sensory cues in addition to sonar. In fact, passive listening plays a major role in prey detection by P. auritus (Anderson & Racey 1991). These bats are exceptionally sensitive to sounds around 15 kHz, which is close to the frequencies emitted by insects moving in clutter (Coles et al. 1989). However, the long-eared bats may also use visual information when searching for prey. In a recent study on feeding behaviour, it was shown that P. auritus preferred to use visual cues to sonar when possible, and that they could detect ca. 2 cm long mealworms visually (Eklöf & Jones, in press). Visual acuity has previously been tested in a number of species (Table 2), both behaviourally by optomotor response tests (Suthers 1966, Bell & Fenton 1986), and theoretically by counting the number of retinal ganglion cells (Koay et al. 1998, Heffner et al. 2001). Both methods give indications of the minimum separable angles, i.e. the minimum distance between two points that an animal need in order to be able to separate them. The acuity values estimated by counting retinal ganglion cells tend to be higher than those estimated from behavioural studies, suggesting that the anatomical method gives a theoretical threshold, rather than what the bats actually respond to. Nevertheless, although the acuity values obtained from the different methods are roughly in the same order, comparisons across the two methods should be made with care. As shown by the literature data presented in Table 2, frugivorous and nectarivorous bats seem to have better spatial resolution than most insectivorous species. Nevertheless, the finest spatial resolution in any bat (3’38’’) is found in the gleaning insectivore Macrotus californicus (Phyllostomidae), and this happens to be the only bat known to find prey, using vision alone (Bell 1985, Bell & Fenton 1986). Indeed, gleaning insectivores may have better visual acuity than aerial-insectivores in general, and this suggests that the aerial-hawking insectivores rely mostly on echolocation rather than vision for detection of small targets, while the opposite may be true in gleaners. At the same time it seems as if, among aerial-hawking insectivores, Emballonuridae have better resolution than Vespertilionidae. The visual resolving power may depend on ambient light intensity. In the common vampire bat Desmodus rotundus, for example, the acuity drops from 48’ at a light intensity of 31 mL (ca. 310 lux) to over 2° in 4*10 -4 mL (ca. 0.004 lux) (Manske & Schmidt 1976). Other bats, such as Macrotus californicus and Antrozous pallidus retain their visual acuity down to light levels as low as 2*10 -4 mL (ca. 0.002 lux) (Bell & Fenton 1986). As a comparison, a light level of 0.1 lux is equivalent to light levels at full moon, and similar to the conditions in this 85
study. On overcast nights the amount of light drops to 0.0001 lux (Ryer1997). Eptesicus fuscus responds optimally to brightness discrimination in ambient light levels around 10 lux (conditions equivalent to dusk or dawn) but performs well down to levels of 0.001 lux (Ellins & Masterson 1974). As the ambient illumination increases towards daylight conditions the visual sensitivity generally declines, although the light tolerance varies between species (Hope & Bhatnagar 1979). Bradbury & Nottebohm (1969) found that Myotis lucifugus avoids obstacles better under ambient illumination resembling dusk than in daylight, which also indicates that the eyes of microchiropteran bats work better in dim light than in bright light. Nevertheless, in a study on optomotor response (Fenton et al. unpublished), several bats responded to striped patterns of 0.9° (the narrowest available in the study) even in bright daylight. Ambient light levels and the way it is measured, if reported at all, varies between different optomotor response studies, which make the results somewhat hard to compare. Acknowledgements I wish to acknowledge Bengt Svensson for building the optomotor device, Lars- Erik Appelquist for making it possible to work at Taberg, Åsa Norén-Klingberg, Jens Rydell, Stefan Pettersson and Karl-Johan Börjesson for help in the field and comments on the manuscript. References Anderson, M. E. & Racey, P. A. 1991. Feeding behaviour of captive long eared-bats, Plecotus auritus. Animal Behaviour 42, 489-493. Baker, A. G. & Emerson, V. F. 1983. Grating acuity of the mongolian gerbil (Meriones unguiculatus). Behaviour and Brain Research 8, 195-209. Bell, G. P. 1985. The sensory basis of prey location by the California leaf-nosed bat Macrotus californicus (Chiroptera: Phyllostomidae). Behavioral Ecology and Sociobiology 16, 343-347. Bell, G. P. & Fenton, M. B. 1986. Visual acuity, sensitivity and binocularity in a gleaning insectivorous bat, Macrotus californicus (Chiroptera: Phyllostomidae). Animal Behaviour 34, 409-414. Bradbury, J. & Nottebohm, F. 1969. The use of vision by the little brown bat, Myotis lucifugus, under controlled conditions. Animal Behaviour 17, 480-485. Chase, J. 1972. The role of vision in echolocating bats. PhD Thesis, University of Indiana,Bloomington. Chase, J. 1981.Visually guided escape responses of microchiropteran bats. Animal Behaviour 29, 708-713. Chase, J. 1983. Differential responses to visual and acoustic cues during escape in the bat Anoura geoffroyi: cue preferences and behaviour. Animal Behaviour 31, 526-531. Chase, J. & Suthers, R. A. 1969. Visual obstacle avoidance by echolocating bats. Animal Behaviour 17, 201-207. 86
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Vision in echolocating bats Johan E
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A doctoral thesis at a university i
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to be precedence of vision over son
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och för att undvika hinder på vä
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B ats (Order: Chiroptera) are among
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VISION IN ECHOLOCATING BATS The mic
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The microchiropteran eyes are shape
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structures, and the dorsal lateral
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10 lux, which is equivalent to the
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The device used for the optomotor r
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Tab 2 - cont Light Visual Species (
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jamaicensis, Desmodus rotundus and
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(Davis & Barbour 1965) or when they
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Insectivores and carnivores For bat
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Moreover, the eyes of Macrotus cali
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combined with broadband echolocatio
Page 35 and 36: Multimodality - vision and echoloca
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Page 39 and 40: Acknowledgements First of all I wou
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Page 73 and 74: Submitted manuscript Vision complem
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